Lag fuse construction and method and apparatus for making same



Sept. 7, 1954 Filed March 6, 1952 G. F. LAING LAG FUSE CONSTRUCTION ANDMETHOD AND APPARATUS FOR MAKING SAME 2 Sheets-Sheet l Patented Sept. 7,1954 LAG FUSE CONSTRUCTION AND METHOD AND APPARATUS FOR MAKING 'SAMEGordon F. Laing, Delavan, Wis., assignor to Economy Fuse andManufacturing 00., Chicago, 111., a corporation of Illinois ApplicationMarch 6, 1952, Serial No. 275,198

4 Claims. 1

This invention relates to the construction and manufacture of lag fuses,and especially those of the type shown and described in my Patent No.2,577,531, issued December 4, 1951.

The invention is primarily directed to construction techniques andapparatus for securing adherence between terminal links and a connectingcentral fusible element which characterize lag fuse constructions ofthis type. The invention also embraces desirable dimensional andvolumetric relationships the better to secure open circuit conditionsonce a fusible element has been melted incident to circuit overload.

Although it has heretofore been the intention to solder terminal linksto the fusible slug in lag 1 fuses, due to the relative low meltingpoints of the metals involved, it has not been easily accomplishedwithout embarking upon the rather costly procedures involved inpreheating and holding the several parts at elevated temperatures for aninterval of time necessary to secure the Welded union.

Since these practices are not regarded practicable under the limitationsof commercial economy, other efforts to connect links to the slugs haveresulted in loose connections which permit the metals to oxidize orcorrode at the interface, aiiording a poor electrical contact andaltering the capacity of the fuse thereby.

In many instances the connection is so loose as to permit the linkelements to withdraw from the fusible slug, with obviously undesirableresults. In this latter connection, the fibrous casings in which suchfuses are assembled are responsive to changes in humidity, so thatconsiderable expansion and contraction under operating conditions andfor substantial intervals .of time subject the link assemblies totension and compression, and in many instancess the forces of tensionare sufiicient to loosen or destroy the contact between the links andfusible slug.

Other problems have arisen in relating the mass of the fusible slugelement to the internal volumetric capacity of the casing in which it isinstalled to insure that the slug once fused on i overload to breakconnection between the link terminals will remainsniiiciently spacedfrom the latter 'eiiectively to maintain open circuit condi- :tion,irrespective of the position of the fuse in use.

In this latter connection there is no problem when fuses of this kindare used in :a vertical-or nearly vertical-attitude, since in such casethe fused metal runs to one end of the interior of the casingsurrounding one of the terminal links where it is adequately spaced fromthe opposite terminal link .to insure open circuit condition.

In the horizontal position, however, especially with the width of theterminal link disposed in a vertical direction, the melted segment can'become dangerously close to the terminal links to incur the risk ofaccidentally maintaininga closed circuit condition beyond the limits ofpractical safety. Hereinafter, certain safeguards against this conditionare disclosed.

Turning now to the drawings:

Figures 1 and 2 are longitudinal mid-sectional views of fuses made inaccordance With the present invention, said sections being taken alongplanes disposed 90 degrees apart;

Figure 3 is a crosss-sectional view regarded along line 33 of Figure 1.;

Figures 4 and 5 are mid-sectional elevational and top plan views,respectively, of a mold designed to give effect to the presentinvention; and

Figures .6 and 7 are end and side diagrammatic views showing certainconsiderations of mass and volume entering into the practice of thepresent invention.

lhe type of lag fuse to which the present invention is directed is shownand described in my patent previously mentioned, so that it will beunnecessary to enter into a detailed description of the constructionshown in Figures 1 to 3 of the drawings hereof, other than briefly toidentify the several parts and their relation to the followingdescription.

In these figures, 1-9 represents a cylindrical tubular fuse casingclosed at each end by end disks ['2 and caps M, both of which areapertured to receive knife-blade terminals l5 in the usual way. The caps14 are secured to the tubular body 46 by suitable fastenings, such asthe screws I8. The interior of the casing is divided into three parts bytwo partitions 20 and 22, respectively, to comprise end compartments '24and 2B, which are usually filled with powder, and a central compartment28. The partitions 20 .and 22 .areslotted centrally to receive fuselinks 30 and Y32, which extend from the internal ends of the knife-bladeterminals l6 into the central compartment 28, where they are connected,in the manner that will later be described herein, to a fusible slug 34.

This invention is concerned with securing a perfect soldered or weldedjoint between the inner ends of the fuse links 30 and .32 and the slug34, which technique will be presently described, and is furtherconcerned with the relation between the mass of the fusible link 34, theinternal volumetric capacity of the central portion 28 of the casing,and the clearance between the slug metal 34 fused into a segment, andthe inner ends of the fuse links 30 and 32, which project into thecentral compartment 28. These latter considerations will be discussed inconnection with Figures 6 and 7.

Turning now to Figures 4 and 5 of the drawing, there is here disclosed amold designated generally at 40, which is composed of two complementaryhalf sections 42 and 44, either one of which may comprise theconstruction shown in Figure 4. Each of the half sections is formed toprovide a principal molding cavity 46 centrally of the mold, and apouring entry 48 which is connected to the principal molding cavity 46by a throat or sprue 59 which is divided near its lower end into twolegs 52 and 54 which enter the cavity 46 proximate to the points atwhich the fuse links 36 and 32 extend therein. The principal mold cavityis connected with an overflow reservoir 56 by a central bottom sprue 58.A central riser 60 having a passageway 62 communicating with theexterior of the mold is provided above and in connecting relation withthe principal mold cavity approximately centrally thereof so as to bleedoff displaced air and gases which would otherwise be entrapped therein.The mold cavity is pierced at each of its ends by slots 64, in which theends of fuse links are adapted to be confined so as to project in equalamounts into the principal mold cavity and substantially beneath thedivided legs 52 and 54 of the pouring entry 50.

The partitions 2E! and 22 for the casing in are first disposed upon theinner extremities of the terminal links, and these latter are theninserted in the apertures fill on opposite sides of the mold 43 so thatthe extremities of the links project oppositely into the mold cavity 46thereof. The mold halves 42 and 44 are then clamped or bolted togetheras by fastening means 66, which may be applied for this purpose.

Fused metal is poured into the mold through the entry 48 of the sprue 5Band legs 52 and 54 so as to flow past the link terminals for a length oftime and in amount sufilcient to fill the overflow reservoir 58, theprincipal mold cavity 46, and their connecting sprue 53. By the timethese cavities are filled, the extremities of the fuse links will havebeen sufiiciently elevated in temperature autogenously to unite theirsurface metal with that contained in the cavity 45. The links areusually first coated with tin solder or similar metal of like purpose,the temperature of which will be adequately elevated by the excess ofmolten slug metal flowing past the fuse link terminals tightly to bondto the latter once the cast is set.

Upon cooling, the mold elements 42 and 44 are separated and the joinedparts removed therefrom, which, of course, include the metal appendageformed by the cavity 56 of the overflow reservoir which must be removedfrom the finished article by cropping and machining, or in any desiredway.

Fuse links made in this manner have exhibited a perfect union betweenthe several parts, possessing the requisite physical and electricalproperties without recourse to heating and holding the mold and itscontents of cast metal and fuse links at elevated temperatures toaccomplish this purpose.

Referring now to Figures 6 and 7, there is diagrammatically illustratedformulae for proportioning the metal volume of fusible slugs similar to34 in the foregoing description to the volumetric capacity of the casingin which it is disposed, similar to the compartment 28 previouslydescribed, so as to maintain open circuit clearance between the fusedsegment of the slug (after overload has melted the same) and the fuselink terminals in the central compartment irrespective of the positionof the fuse assembly.

As shown, the slug 34 is represented as being formed to radius 1" andthe internal radius of the compartment 28 is indicated at R, and itslength is indicated by l in Figure '7 Manufacturing considerationsentering into the production of the mold 40 make desirable forming thecavity 46, in which the slug 34 is shaped, with concave hemisphericalends, which, as shown in Figure 2, are scribed to radii n, which areequal to the radius of the slug r as shown in Figure 6. Otherwise, theslug may be square ended or conical at these portions. The cylindricalportion of the slug between the hemispherical ends of the exampleillustrated is indicated by the dimension h in Figure 7, and theclearance C between the fused segment of the slug 35 as shown in Figure6 and the terminal of the fuse link is pre-established to afford thenecessary clearance for the retention of open circuit conditions.

Now with these pre-established conditions, the correct relationshipbetween the parts can be determined from the following, which are givenon the second sheet of the drawings hereof, when the formulae areexpressed as cubic equations:

(1) Volume of cylindrical part of slug=pi r h (2) Volume of sphericalparts of slug=4.l89r

(being two hemispheres in which r =r) 0 in degrees is obtainedgraphically Where R and C are established.

h and l are selected. Solve for r.

(4) Volume of segment:

(5) (pi Wad-4.189%):

The foregong formulae, when expressed and solved as cubic equations,determine the diametric size of the fusible slug 34 and assure theproper clearance between segment and terminals in all positions of thefuse. Of course, the minimum proportions of the slug B must be that masswhich affords the necessary capacity for the circuit with which it isassociated. The maximum proportions must be those in which necessaryclearance between the segment of the fused slug and the terminals ismaintained to insure continued open circuit conditions in any positionof the fuse in accordance with the preceding formulae.

I claim:

1. A lag fuse comprising a cylindrical casing divided into threecompartments, terminals at each end of said casing, terminal linksextending from said terminals across the end compartments, respectively,and terminating in the intermediate compartment, a slug of cast fusiblemetal autogenously united to the opposed end of said links within theintermediate compartment and being inseparably bonded thereto to affordoptimum physical and electrical connection between said links attemperatures below its melting point.

2. The fuse construction of claim 1, in which said slug is formed withsubstantially hemispherical ends and is formed to a radius 7 determinedby the following equation to afiord the requisite clearance C betweenthe link ends which terminate in the intermediate compartment when thefuse is in a horizontal attitude with said links disposed with theirgreatest dimension of breadth substantially vertical and the metalsegment resulting from the slug when melted:

2 3 pi R 01 (p1rh+4.189r) 360 said links outside of said mold, castingfused metal in said mold in sufi'icient mass autogenously to unite thecast metal with said links so as permanently to connect the lattertogether between the disk-like elements, relating the cubic volume ofthe space between said disk-like elements to the mass of cast metaluniting said 6 links to provide a fuse which will afford open circuitclearance after the cast metal has melted to disunite said terminallinks irrespective of the position of the fuse when melting occurs.

4. The method of claim 3, in which open circuit clearance isaccomplished by equating the mass of cast metal in terms of its volumewith the cubic volume of said space according to the formula:

Total volume of cast metal:

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 878,869 Cole Feb. 11, 1908 1,669,071 Thomas May 8, 19282,486,798 Mollenhauer Nov. 1,4949 2,561,464 Cremer July 24, 19512,577,531 Laing Dec. 4, 1951

